Describe the menstrual cycle hormonal control

Overview

The menstrual cycle is a ~28-day recurring process controlled by a negative feedback loop involving the hypothalamus, pituitary gland, and ovaries. Four key hormones orchestrate this cycle: GnRH (gonadotropin-releasing hormone), FSH (follicle-stimulating hormone), LH (luteinizing hormone), and the ovarian hormones estrogen and progesterone.

The Hypothalamic-Pituitary-Ovarian (HPO) Axis

This axis operates via feedback loops: ovarian hormones signal back to the brain to adjust FSH/LH output.

The Four Phases and Hormonal Changes

Phase 1: Menstruation (Days 1–5)

What happens: The uterine lining (endometrium) sheds because the previous cycle's corpus luteum died, cutting off progesterone supply.

Hormonal state:

• Progesterone and estrogen are LOW (corpus luteum has degraded)
• FSH begins to RISE slightly (low estrogen/progesterone removes negative feedback on pituitary)

WHY the bleeding? Progesterone maintains the endometrium by keeping blood vessels stable. When progesterone drops, blood vessels constrict → tissue dies → menstrual flow.

Phase 2: Follicular Phase (Days 1–13)

What happens: Multiple ovarian follicles begin maturing under FSH stimulation. One dominant follicle outcompetes the others.

Hormonal dynamics:

1. FSH rises (days 1–7): Stimulates follicle growth
2. Growing follicles secrete estrogen (days 7–13):
   • Estrogen rebuilds the endometrium (proliferation)
   • Low-moderate estrogen → negative feedback on FSH (FSH drops slightly)
   • High estrogen (day 12–13) → positive feedback on LH (triggers LH surge)

WHY does estrogen switch from negative to positive feedback?

• Low estrogen (early follicular): Acts as a brake — "Don't ovulate yet, eggs aren't ready"
• High estrogen (late follicular): Acts as a trigger — "Egg is mature, GO for ovulation!"

This switch occurs at a threshold of ~200 pg/mL estrogen sustained for ~48 hours.

Phase 3: Ovulation (Day 14)

What happens: The LH surge causes the dominant follicle to rupture and release the ocyte (egg).

Hormonal trigger:

High estrogen from the dominant follicle flips the pituitary switch:

1. Estrogen → hypothalamus releases MORE GnRH (pulses faster)
2. GnRH → pituitary releases massive LH spike (10x baseline in 24h)
3. LH surge → follicle wall weakens → egg bursts out (~24–36h after surge peak)

WHY does LH cause rupture? LH activates enzymes (colagenase, plasmin) that digest the follicle wall. Think of it as LH "unlocking the door" for the egg.

Phase 4: Luteal Phase (Days 15–28)

What happens: The ruptured follicle transforms into the corpus luteum ("yellow body"), which secretes progesterone and some estrogen.

Hormonal state:

• Progesterone HIGH (peaks ~day 21)
• Estrogen moderate
• LH/FSH LOW (negative feedback from P and E)

Progesterone's jobs:

1. Maintains endometrium: Converts proliferative lining → secretory lining (rich in glycogen, ready for implantation)
2. Raises body temperature (thermogenic effect)
3. Inhibits new ovulation: Negative feedback on GnRH/LH/FSH

If fertilization occurs: $\text{Embryo secretes hCG} \rightarrow \text{hCG rescues corpus luteum} \rightarrow \text{P stays high} \rightarrow \text{pregnancy}$

WHY does the corpus luteum die? Without hCG (human chorionic gonadotropin) from an embryo, the corpus luteum has a programed 14-day lifespan. LH alone can't sustain it long-term. When it dies → progesterone plummets → endometrium destabilizes → menstruation → cycle restarts.

The Feedback Loop Summary

2. Estrogen (early-mid follicular): $\text{Rising E} \rightarrow \text{Inhibits FSH} \rightarrow \text{Suppresses non-dominant follicles}$

WHY is positive feedback rare in biology? Most systems use negative feedback for stability (like a thermostat). Positive feedback is explosive — used only when you WANT a rapid, dramatic event (ovulation). Once the follicle ruptures, estrogen drops, shuting off the positive loop.

Derivation: Why Does FSH Drop Mid-Cycle?

Let's derive this from feedback principles.

Given:

• FSH stimulates follicle growth
• Follicles produce estrogen
• Estrogen inhibits FSH (negative feedback)

Step 1: Early cycle (days 1–5): Low estrogen → low inhibition → FSH rises

Step 2: Mid cycle (days 7–12): Growing follicles → estrogen rises → estrogen binds to pituitary receptors → reduces FSH release

Step 3: Mathematically, if FSH production rate is $k_1$ and estrogen inhibition is proportional to $[E]$:

$\frac{d[\text{FSH}]}{dt} = k_1 - k_2 [E] - k_3 [\text{FSH}]$

where $k_3$ is natural clearance.

At equilibrium ($\frac{d[\text{FSH}]}{dt} = 0$):

$[\text{FSH}]_{\text{steady}} = \frac{k_1 - k_2 [E]}{k_3}$

As $[E]$ increases (follicles grow), $[\text{FSH}]_{\text{steady}}$ decreases. This is why FSH peaks early (day 3) and drops as estrogen rises (days 7–13).

Step 4: But LH surges despite high estrogen — WHY? The pituitary has two receptor populations:

• Low-dose E receptors → inhibit LH (negative feedback)
• High-dose E receptors → stimulate LH (positive feedback, only activated >200 pg/mL)

This dual-receptor model explains the switch.

Common Mistakes

The flaw: At very high levels (late follicular phase), estrogen stimulates LH via positive feedback. The dose and duration matter.

The fix: Remember the threshold rule: <200 pg/mL = negative feedback; >200 pg/mL sustained = positive feedback.

The flaw: FSH actually drops before ovulation. It's the LH surge (not FSH) that causes the follicle to rupture.

The fix: FSH = "grow the follicles." LH = "release the egg." Different jobs.

The flaw: It's not new — it's the transformed follicle after ovulation. Same cells, new function (now secreting P instead of E).

The fix: Think of it as the follicle's "second career."

Key Formulas

Mnemonics & Recall

Week 1 (Menstruation): The body cleans out the old room because no baby came last month.

Week 2 (Follicular): A tiny egg starts growing in the ovary. As it grows, it sends a signal (estrogen) that tells the uterus, "Start decorating the room!" The uterus walls get thick and soft.

Week 3 (Ovulation): The egg is ready! A big signal (LH surge) tells the egg, "Time to leave the ovary!" The egg pops out and travels to the uterus.

Week 4 (Luteal): The ovary now makes a new signal (progesterone) that says, "Keep the room cozy and warm." If the egg meets a sperm and makes a baby, it moves into the cozy room. If not, the body thinks, "No guest this month," stops making progesterone, and cleans out the room again (back to Week 1).

The brain is like the manager giving instructions, the ovary is the worker making the egg and signals, and the uterus is the room being prepared.

Worked Examples

Step 1: Check estrogen threshold.

• Estrogen >200 pg/mL sustained → positive feedback activated

Step 2: Predict LH.

• High estrogen → LH surge imminent (should rise to 40–80 mIU/mL)

Step 3: Predict timing.

• LH surge → ovulation in 24–36h

Answer: Ovulation will occur in ~1.5 days. She's in the late follicular phase.

Why this step? The 200 pg/mL threshold is the critical switch point where estrogen's effect flips from inhibition to stimulation of LH.

Step 1: Trace the ovulation pathway.

• Ovulation requires: High E → GnRH surge → LH surge → follicle rupture

Step 2: Identify the break.

• High E is present, but no LH surge → problem is in the pituitary or hypothalamus response to estrogen

Step 3: Possible causes.

• Pituitary damage (can't make LH)
• Hypothalamic dysfunction (GnRH neurons not responding)
• Medications suppressing GnRH (e.g., stress, PCOS, hypothalamic amenorrhea)

Answer: This is anovulation due to HPO axis dysfunction. The follicle grows but never ruptures.

Why this step? By systematically checking each step of the cascade, we pinpoint where the signal chain breaks.

Step 1: Normal cycle expectation.

• If no pregnancy, corpus luteum dies by day 28→ P drops → menstruation

Step 2: Observed data.

• P is HIGH (corpus luteum still alive)
• hCG is present (only from embryo)

Step 3: Conclusion.

• The embryo implanted → secretes hCG → hCG rescues corpus luteum → P stays high → endometrium maintained → no period

Answer: She's pregnant (~2 weeks post-ovulation). The corpus luteum is being sustained by embryonic hCG.

Why this step? hCG structurally mimics LH, so it binds to LH receptors on the corpus luteum, extending its lifespan beyond14 days.

Connections

• Hypothalamus and Pituitary Hormones – GnRH, FSH, LH secretion mechanisms
• Steroid Hormone Synthesis – How ovaries make estrogen and progesterone from cholesterol
• Negative and Positive Feedback in Endocrine Systems – General principles
• Ovarian Follicle Development – Detailed folliculogenesis
• Pregnancy Hormones – hCG, placental hormones
• Disorders of Menstruation – Amenorrhea, PCOS, luteal phase defects
• Oral Contraceptives Mechanism – How synthetic hormones prevent ovulation

#flashcards/biology

What is the HPO axis? :: The hypothalamic-pituitary-ovarian axis: hypothalamus releases GnRH → pituitary releases FSH/LH → ovaries release estrogen/progesterone, with feedback loops controlling the cycle.

Concept Map

Hinglish (regional understanding)